Methods for making fixed parallel plate MEMS capacitor microsensors and microsensor arrays

ABSTRACT

A fixed parallel plate micro-mechanical systems (MEMS) based sensor is fabricated to allow a dissolved dielectric to flow through a porous top plate, coming to rest on a bottom plate. A post-deposition bake ensures further purity and uniformity of the dielectric layer. In one embodiment the dielectric is a polymer. In one embodiment, a support layer is deposited onto the top plate for strengthening the sensor. In another embodiment, the bottom plate is dual-layered for a narrowed gap. Integrated circuit arrays of such sensors can be made, having multiple devices separated from each other by a physical barrier, such as a polycrystalline containment rim or rough, for preventing polymer material from one sensor from Interfering with that of another.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to chemical microsensors. Moreparticularly, the present invention is directed to fixed parallel platechemical microsensors and microsensor arrays and methods of making same.

2. Background Information

Sensors based on micro electro-mechanical systems (MEMS) devices areuseful for sensing a wide range of chemical conditions, such as thepresence of volatile organic compounds (VOCs), hydrocarbon gases, andother analytes. Manufacturing techniques for building MEMS sensors,especially integrated circuit arrays of such sensors, usually involvemany fabrication steps. Unfortunately most, if not all, of the sensorfunctionality has to be built into the microchip up front making itdifficult to customize or tune a sensor to fit a particular need aftermanufacture. For example, the dielectric material of a parallel platecapacitor sensor is usually deposited onto a first conductive electrodeduring an intermediate phase of the fabrication process, followed bydeposition of a top plate electrode. Furthermore, MEMS devices aretypically manufactured with solid dielectric materials whose physicalproperties require that layers be deposited in a specified order.

Other MEMS device configurations can be used as well to sense chemicals,for example, by measuring the capacitance changes of chemicallysensitive materials with an inter-digitated finger capacitor or changesin physical properties with a cantilever. However, these arrangementsoften call for semi-rigid or flexible conductors that suffer fromseveral disadvantages, including excess stray capacitance, stiction, andsensitivity to vibrations.

SUMMARY OF THE INVENTION

The devices and methods of the present invention overcome theaforementioned disadvantages by providing a fixed parallel platecapacitor microsensor that comprises a porous top plate, a bottom plate,and a chemically absorbent dielectric material in between. In oneembodiment, the dielectric material is a dissolved polymer depositedonto the porous top plate and permitted to flow through the pores,creating a uniform coat on the bottom plate and substantially filling anetched sensing gap created between the plates by a previous process.

In another embodiment, the dissolved polymer dielectric is depositedonto the device by spin coating, spray coating, or dip coating. In yetanother embodiment, the dissolved polymer dielectric is baked in an ovenafter deposition. In still another embodiment, one or more supportlayers is deposited on the device for added structural support.

In one embodiment, a plurality of fixed parallel plate capacitormicrosensors is micromachined onto a substrate to create an array ofdevices for varying the sensitivity and/or selectivity of the array.Included in this embodiment is a network of containment barriers, suchas a rim or a trough, for containing dielectric material betweendevices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a fixed parallel plate capacitorsensor having a support layer and designed in accordance with thedevices and methods of the invention.

FIG. 2 is a top-level view of a circular-shaped fixed parallel platecapacitor sensor having no support layer and implemented according to anembodiment of the invention.

FIG. 3 is a cross-sectional view along A-A′ of FIG. 2.

FIG. 4 is a top-level view of a rectangular-shaped fixed parallel platecapacitor sensor having a support layer and implemented according to anembodiment of the invention.

FIG. 5 is a cross-sectional view along A-A′ of FIG. 4.

FIG. 6 shows a top-level view of a rectangular-shaped fixed parallelplate capacitor sensor having a pattern of slotted top plate pores andimplemented according to an embodiment of the invention.

FIG. 7 is a cross-sectional view of a fixed parallel plate capacitorsensor having a dual-layer sealed bottom plate in accordance with thedevices and methods of the present invention.

FIG. 8 is a top-level view of a rectangular-shaped fixed parallel platecapacitor sensor having a dual-layer sealed bottom plate and implementedaccording to an embodiment of the invention.

FIG. 9 is a cross-sectional view along A-A′ of FIG. 8.

FIG. 10 is a flow diagram illustrating a method for depositing a polymerdielectric between two fixed parallel plates of a capacitor sensoraccording to an embodiment of the invention.

FIG. 11 illustrates a sample containment barrier implemented as a rim inaccordance with the devices and methods of the invention.

FIG. 12 is an exemplary view of a rim implementation of a containmentbarrier having three polycrystalline semiconductor layers.

FIG. 13 exemplifies a fixed parallel plate sensor array comprising aplurality of capacitor sensors separated from each other by a network ofcontainment barriers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The devices and methods of the present invention overcome the drawbacksinherent with current MEMS devices and fabrication techniques byproviding a fixed parallel plate capacitor that reduces or eliminatessuch operational side effects as dielectric swelling. In addition,fabrication of a parallel plate capacitor sensor is made more flexibleby allowing dielectric deposition to occur after the device ismanufactured. This allows preparation of sensors with many differentdielectric properties and materials characteristics. The dielectricmaterial flows through the openings of a porous top plate, coming torest on the device bottom plate, as further described to follow.

FIG. 1 is a cross-sectional view of a fixed parallel plate capacitorsensor 100 having a porous top plate electrode and designed inaccordance with the devices and methods of the present invention.Capacitor sensor 100 can be easily manufactured using standardphotolithographic techniques, such as by creating vias where individuallayers require contact or by using hydrofluoric acid to etch awaysacrificial layers. Capacitor sensor 100 is a layered device comprisingbottom plate 103, porous top plate 110, dielectric 107, and built onsubstrate 101 to be electrically isolated therefrom by isolation layer105. Support layer 112 is deposited onto porous top layer 110 to providestrength to capacitor sensor 100.

Dielectric 107 is made from a chemically absorbent material that changespermitivity in the presence of a chemical, such as a target analyte. Inone embodiment, dielectric 107 is a non-conductive material such as apolymer, solgel, or ceramic. In other embodiments, dielectric 107 ismade of a semi-conductive material, such as a composite of a polymer andan ionic or electronic conductor.

In one embodiment, bottom plate 103 is a conductive material, such aspolycrystalline silicon. In another embodiment, bottom plate 103 is aconductive material, such as a metal. Similarly, porous top plate 110 isin one embodiment a conductive material, such as polycrystallinesilicon, while in other embodiments, top plate 110 is made of aconductive material such as a metal.

Top plate 110 is porous in order to permit the cavity between top plate110 and bottom plate 103 to be coated with a viscous dielectric fluidduring a deposition process according to the methods described herein.Top plate 110 can be made of a conductor that is innately porous, or topplate 110 can be made porous during fabrication, by patterning injectionholes across the plate surface.

Support layer 112 can be a conductive material such as polycrystallinesilicon, or other flexible or semi-rigid material that prevents devicesub-layers from collapsing or bending. In one embodiment, support layer112 is made from polycrystalline silicon. Support layer 112 can bepatterned with gaps 114 separating the support layer from top plate 110,as shown or support layer 112 can be a solid layer, contacting top plate110 of capacitor sensor 100 over the entire surface of top plate 110,for added strength. Support layer 112 can be deposited onto too plate110 by etching away vias at those locations on top plate 110 wherecontact between top plate 110 and support layer 112 is desired.

FIG. 2 is a top-level view of a circular-shaped fixed parallel platecapacitor sensor 200 implemented according to an embodiment of thepresent invention. In this implementation, capacitor sensor 200comprises a network of pores 201 interspersed across the surface oftop-plate 110, thus creating the porosity needed to allow dielectricinjection onto bottom plate 103, escape paths for the displaced air, andto further permit absorption of atmosphere later during deviceoperation. In one embodiment, central fill hole 205, generally larger insize than pores 201, is used to permit drops of dielectric to floweasily underneath top plate 110 during dielectric

An array of anchors 204 affixes top plate 110 to substrate 101 andprevents collapse of top plate 110 onto bottom plate 103. Anchors 204are provided for support and can be placed across the surface of thecapacitor in many different configurations, including the configurationshown in FIG. 2 having four anchors near to and surrounding fill hole205. In a typical case, the outer perimeter 209 of capacitor 200 alsocomprises a continuous anchor around the device perimeter to providesupport at the edges. Anchors 204 provide device stiffness to helpcounteract polymer swelling during sensor operation, and are chieflyresponsible for making porous top plate 110 fixed with respect to bottomplate 103. Anchor 204 can be vias made by etching away the samesacrificial layer that produces the sensing gap between top plate 110and bottom plate 103.

Bottom plate 103 in this implementation is a drive electrode that spansthe bottom surface of the device, the continuity of which is interruptedonly at those points where top plate 110 is pierced by an anchor 204through to substrate 101. Bottom plate 103 is fabricated to electricallyconnect to a bottom plate electrode 215 while top plate 110 isfabricated to electrically connect to a top plate electrode 218. Bottomelectrode 215 acts as a common lead for capacitor sensor 200 incompleting a sensing circuit. Top electrode 218 is typically attached toa bonding pad for connectivity to a readout chip capable of measuringcapacitance.

FIG. 3 is a cross-sectional view along A-A′ of FIG. 2 showing the layersthat comprise capacitor sensor 200. The continuity of bottom plateelectrode 215 is interrupted by anchors 204 that pierce through bottomelectrode 215 making contact with isolation layer 305. Fill hole 205provides access to bottom plate electrode 215 for injection ofdielectric into the sensing gap region 307 left vacant by the etching ofa sacrificial layer.

FIG. 4 is a top-level view of a rectangular-shape fixed parallel platecapacitor sensor having a support layer and implemented according to analternative embodiment of the present invention. In-this implementation,capacitor sensor 400 comprises pores 401 interspersed across the surfaceof top-plate 110 in a checkered fashion. Anchors 404 are evenly spacedacross the device surface, providing extended rigidity between top plate110 and bottom plate 103.

FIG. 5 is a cross-sectional view along A-A′ of FIG. 4 showing the layersthat comprise capacitor sensor 400. The large number of anchors 404 inthis embodiment permit a smaller sensing gap 550 than the sensing gap350 created by the previous embodiment of FIGS. 2 and 3. Laboratoryresults have produced a device of this nature with a sensing gap 550 of2.0 microns.

The larger number of pores 401 in top plate 110 can make dielectricinjection easier in some cases, as well as offer more efficient chemicalabsorption during device operation. In other cases, however, a strongpolymer/solvent interaction chemistry will make a circular fill hole 205a more efficient structure for forcing a mildly viscous liquiddielectric through pores. The grid-shaped support layer 412 further addsstrength to the sensor in this embodiment. The perimeter of the devicecan be perforated to allow for more efficient filling by allowing moreair escape holes.

A variety of different top-plate pore configurations and device anchorarrangements is contemplated by the present invention. FIG. 6, forinstance, illustrates yet another top-level view of a rectangular-shapedfixed parallel late capacitor sensor 600 having a pattern of pores intop plate 110 fashioned using a series of slots 601. Fewer anchors 604are needed in this embodiment because of the support characteristics ofthe slotted pores.

Many factors contribute to the overall design goal of providing a pairof plates, parallel in orientation, which maintain their rigidity as thepolymer dielectric begins to swell under atmospheric condition. Ingeneral, a trade-off exists between the strength provided by a tighterpattern small pores and the ease with which polymer dielectric injectedunderneath top plate 110. More pores in top plate 110 can mean moreanchors will be needed between plates to negate the flexibility of thetop plate and the effects of swelling. The pare geometry of the devicethus is an important factor in meeting design constraints.

FIG. 7 is a cross-sectional view of a fixed parallel plate capacitorsensor having a dual-layer sealed bottom plate according to the devicesand methods of the present invention. Capacitor sensor 700 is a layereddevice comprising dual-layer bottom plate 703, porous top plate 710,dielectric 707, and built on substrate 701 to be electrically isolatestherefrom by isolation layer 705. Support for device 700 issubstantially provided by dual-layer sealed bottom plate 703 in thisembodiment. Bottom plate 703 comprises first bottom layer 704 depositedonto isolation layer 705, followed by a second bottom layer 709, whichtogether form a sealed area 718 between the two bottom layers. The netresult is a raised bottom plate, which reduces the gap distance. Thistype of device allows different gaps for various arrayed devices in asingle process. Narrow gaps may be necessary or at least advantageousfor low-dielectric or low-sensitivity polymers.

FIG. 8 is a top-level view of the fixed parallel plate capacitor sensor700 of FIG. 7 having a dual-layer sealed bottom plate 703 andimplemented according to an embodiment of the present invention. Fromthis view are visible the larger anchor regions 804 contemplated by thisembodiment. Larger anchors 804 account for the added strength of thisembodiment, which in turn permit a smaller sensing gap between top plate710 and bottom plate 703 without requiring an added support layer suchas 112.

FIG. 9 is a cross-sectional view along A-A′ of FIG. 8 showing the layersthat comprise capacitor sensor 800 showing both the smaller sensing gap950 and the sealed areas 718 between bottom layers 704 and 709.Laboratory results have produced a device of this nature with a sensinggap 950 of 0.75 microns.

FIG. 10 is a flow diagram illustrating a method for depositing a polymerdielectric between two fixed parallel plates of a capacitor sensor inaccordance with one embodiment of the invention. The process begins withstep 1000 where a pre-fabricated, micro-machined fixed parallel platecapacitor sensor is delivered from a previous process and whose sensinggap has not yet been filled with sensing material. In step 1001, asensor, such as the sensor of FIG. 2, is prepared for polymerdeposition. Preparation details depend largely on the particularlaboratory processes to be used in the following steps. For instance,preparation step 1001 can include making any electrical connectionsnecessary to effectuate monitoring of the capacitance in steps 1004 and1005 to follow, as well as dicing the wafer containing the sensordevices. In step 1002, a polymer dielectric to be deposited between theplates of the sensor is dissolved into a solvent. The solvent can be,for instance, chloroform, water, acetone, toluene, methanol, or anyother suitable solvent that will permit the polymer/solvent mixture toflow through a fill hole 205 and/or pores 201 in porous top plate 110. Atypical polymer concentration in solution is between 0.075 percent and 1percent by weight.

In step 1003, the polymer/solvent drops are deposited onto or near anyone of the pores created in top plate 110. In the embodiment of FIG. 2,polymer injection is easily accomplished by depositing one or more dropsof polymer over the larger, central injection hole. For a highly viscouspolymer/solvent mixture, injection of dielectric material throughinjection hole 205 or top plate pores 201 can be accomplished by aninkjet-style injector. For larger drops, such as are characteristic of asolid polymer, a syringe can be used for polymer injection. Using eithertechnique, the goal is a complete polymer coating of bottom plate 103.To achieve a complete polymer coat, inkjet injection usually requiresmore drops than a syringe because inkjet nozzles are capable ofdelivering a smaller sized drop. Lab results have shown that an inkjetprocess can take up to 1000 drops to achieve complete coverage, whereasa syringe can take as few as 5 comparatively larger drops.

The drops are allowed to filter down through pores 201, coming to restonto bottom plate 103 in the form or a uniform coat over the surface ofbottom plate 103 and filling sensing gap 350 created thereunder by theetch of a sacrificial layer during the previous sensor fabricationprocess of step 1000. In one embodiment, polymer is drawn into sensinggap region 307 by capillary action, displacing the air and changing thedielectric properties of the capacitor sensor in the process. In anotherembodiment, a spin coat, spray coat, or dip coat is contemplated forachieving greater uniformity of the polymer layer. As a byproduct ofdeposition, the solvent will evaporate, leaving behind a wet layer ofpolymer.

In step 1004, the capacitance between top plate 110 and bottom plate 103is measured and used to monitor and control the deposition of polymer instep 1003. If the measured capacitance in step 1004 is increasing, thendeposition continues with more drops of polymer solution added (step1003) until the measured capacitance stabilizes or reaches a desiredvalue (step 1005). Once a stable capacitance is measured between topplate 110 and bottom plate 103 in step 1005, then polymer depositionstops and processing continues with step 1006.

In step 1006, the device is passed through a post-deposition bakewherein the polymer is dried and readied for back-end processing andpackaging in step 1007. Bake step 1006 will dry away remaining solventthat did not evaporate during deposition step 1003. Post-deposition bakeusually occurs in an oven at a temperature of about 110 degreesCentigrade and lasts between 10 and 60 minutes. Bake step 1006 isparticularly useful for softening a solid polymer so that the polymeruniformly fills the sensor gap and displaces as much air as possible.

Excess polymer can sometimes runoff during deposition step 1003 andtherefore needs to be contained in order to prevent polymer escaping thedevice boundary. Polymer run-off is particularly acute when makingdevice arrays where run-off can interfere with a nearby device.Containment is accomplished by strategic placement of containmentbarriers to prevent dielectric deposited onto one sensor frominterfering with the dielectric of another sensor. In one embodiment,polymer containment is accomplished using a network of troughs to trapexcess run-off during polymer deposition. In another embodiment, polymercontainment is accomplished using a network of rims to isolateneighboring sensors, as explained to follow.

FIG. 11 illustrates a sample containment barrier implemented as a rim inaccordance with the devices and methods of the present invention. Rim1100 is built on a portion of substrate 1101 between neighboringcapacitor sensor devices and electrically isolated therefrom byisolation layer 1106. In one embodiment, the rim is made from apolycrystalline semiconductor, such as polycrystalline silicon,preferably co-deposited with an existing layer so as to avoid redundancyin fabrication. FIG. 12 is an exemplary view of a rim implementation ofa containment barrier having three polycrystalline semiconductor layers.Rim 1200 is built on a portion of substrate 1201 between neighboringcapacitor sensor devices and electrically isolated therefrom byisolation layer 1205. In this embodiment, containment rim 1200 comprisesthree polycrystalline semiconductor layers, each layer of rim 1000 ispreferably co-deposited contemporaneously with a layer duringfabrication or the sensor of FIG. 2. Layer 1203 of containment barrier1200 is co-deposited during deposition of bottom plate 103. Likewise,layer 1210 is co-deposited during deposition of porous top plate 110 andlayer 1212 is co-deposited during deposition of support layer 112. Aswith the sensor devices themselves, co-deposition of layers 1203, 1210and 1212 comprising containment barrier 1200 can use standardphotolithography.

In one embodiment of the present invention, a fixed parallel platecapacitor sensor of the kind disclosed herein can be manufactured as anarray of devices. FIG. 13 exemplifies a fixed parallel plate sensorarray 1300 comprising a plurality of capacitor sensors 400 separatedfrom each other by a network of containment barriers 1200. A sensorarray is advantageous for several reasons. First, by manufacturing eachdevice dielectric to have an incrementally different dielectricconstant, an array can be tuned to sense those chemicals whose behaviorchanges in accordance with varying dielectric properties. Second, eachdevice in an array could be manufactured with a different dielectricmaterial, thereby permitting the array to sense multiple differentchemicals. Thus, varying the selected material and/or dielectricconstant of each device in an array can increase chemical sensitivityand/or improve selectivity of the array. Such a sensor array can becomprised of a variety of sensor devices.

While the particular devices and methods herein shown and described indetail are fully capable of attaining the above described objects of thethis invention, it is to be understood that the description and drawingspresented herein represent a small number of embodiments of theinvention and are therefore representative of the subject matter whichis broadly contemplated by the present invention. It is furtherunderstood that the scope of the present invention fully encompassesother embodiments that may become obvious to those skilled in the artand that the scope of the present invention is accordingly limited bynothing other than the appended claims.

1-29. (canceled)
 30. A method of making a sensor, comprising: a.providing a fixed parallel plate capacitive sensor comprising a bottomplate electrode disposed on a substrate and a porous top plate electrodein fixed, spaced relation to the bottom plate electrode, wherein acavity exists between the bottom plate electrode and top plate electrodeprior to deposition of a dielectric material; and b. depositing adielectric material through the porous top plate electrode to fill thecavity the bottom plate electrode and top plate electrode, therebyforming the sensor.
 31. A method according to claim 30, furthercomprising dissolving the dielectric material in a solvent prior to itsdeposition.
 32. A method according to claim 31, wherein the solvent isselected from the group consisting of water, toluene, chloroform,methanol, and acetone.
 33. A method according to claim 31, wherein theconcentration of the dissolved dielectric material is in the range ofbetween about 0.075 percent and one percent by weight.
 34. A methodaccording to claim 30, further comprising baking the sensor.
 35. Amethod according to claim 34, wherein the baking process ranges fromabout 10 minutes to about 60 minutes.
 36. A method according to claim34, wherein the baking process comprises heating the sensor to atemperature of about 110° C.
 37. A method according to claim 30, furthercomprising depositing one or more support layers on to the porous tipplate electrode.
 38. A method according to claim 37, wherein the one ormore support layers comprises a patterned polycrystal.
 39. A methodaccording to claim 30, further comprising terminating the deposition ofthe dielectric material based upon a capacitance measure between theporous top plate electrode and the bottom plate electrode.
 40. A methodaccording to claim 30, wherein the dielectric material is selected fromthe group consisting of a polymer, a ceramic, a zeolite, and a solgel.41. A method according to claim 30, wherein at least one of the poroustop plate electrode or the bottom plate electrode is comprised of apolycrystalline silicon or a metal.
 42. A method according to claim 30adapted for making a sensor array, wherein the array comprises aplurality of sensors disposed on a substrate, at least one of whichsensors is a fixed parallel plate capacitive sensor.
 43. A methodaccording to claim 42, wherein at least two of the sensors of the sensorarray are fixed parallel plate capacitive sensors.
 44. A methodaccording to claim 43, wherein a containment barrier is disposed betweenat least two of the fixed parallel plate capacitive sensors disposed onthe substrate.
 45. A method according to claim 44, wherein eachcontainment barriers is adapted to prevent leakage of the dielectricmaterial of one fixed parallel plate capacitive sensor to another fixedparallel plate capacitive sensor during fabrication.
 46. A methodaccording to claim 44, wherein a containment barrier is comprised of amaterial selected from the group consisting of a polymer and a metal.47. A method according to claim 44, wherein the containment barrier isshaped as a rim or a trough.
 48. A method according to claim 43, whereineach of the fixed, parallel plate capacitive sensors comprises adifferent dielectric material.
 49. A method according to claim 30,wherein the bottom plate electrode and porous top plate electrode arefabricated using standard photolithography.